Very simple: your department spends far more on it brings in, and taxpayers cover the large gap.
Which is fine; that's how government services always work. But don't pretend that what you do is paid for by something that you produce.
Don't kid yourself - your value is not higher than what the private market is willing to pay you. In fact that is the very definition of value. Public unions have ways of twisting and distorting that, though, precisely because they people the unions are negotiating with are making concessions with other people's money, so it's much easier to get them to pay for things that they a private corporation simply couldn't afford, and would have to refuse.
I already explained why private providers make less money. Privates don't have an endless well of taxpayer money to draw from like public agencies do.
I also asked before why more private agencies aren't represented by large unions, and you never answered. The reason is because the unions 1. aren't eager to represent private agency employees, because there is little money to be made (because there isn't that endless well of taxpayer dollars that I keep pointing out), and 2. when unions are brought in, they don't increase compensation much (lack of that endless well of taxpayer dollars again), so employees see it as more of a hassle than it's worth.
A union strategically and intentionally brainwashes it's members into having an overinflated sense of entitlement and worth (which isn't hard to do, since everyone wants to think they are worth more than they are getting), and then uses political pressure, poorly-informed public opinion, and lack of transparency to pressure local officials into approving compensation expenditures that are often far higher than what the market would allow for, should customers actually have to pay directly for service. And then the union itself skims off the top, like a leach. That's how public unions work. It's not good or bad (well, in some cases it's very bad); it's just how it works. Privates unions, again, are very different.